Incandescent lights work in a full cycle of sinusoid AC voltage, so there is no concern about their polarities when connecting them to either a DC or an AC power line. An LED light, however, being a diode, conducts current only in one direction, i.e. from its anode side to cathode side. To work properly, an LED light must be connected with a right polarity in a DC power line, where a positive voltage must be applied from anode to its cathode. When an LED is used in an AC circuit, it conducts current only for half cycle of the AC voltage, i.e. only when the AC voltage has a positive voltage offset from the LED anode pin to the LED cathode pin.
When two or more LEDs are connected in series, all LEDs must be connected in a same polarity direction, i.e. the anode of the second LED must be connected to the cathode of the first LED, and the anode of the third LED must be connected to the cathode of the second LED, and so on. Otherwise, no current can flow through the series circuit, if one or more LEDs are connected in an opposite polarity direction with respect to the rest LEDs.